Single component adhesive with an adaptable open joint time

ABSTRACT

The invention relates to a one-component epoxy resin adhesive which is elasticated with silicone rubber and whose open joint time is adaptable to whichever production process it is being used in by the concentration of UV initiator and/or the duration and/or intensity of UV activation and which is therefore particularly suitable for automatable manufacturing processes, since the workpieces after initial fixing require no additional mounts even during thermal aftercuring. This adhesive is preferentially suitable for the extensive adhesive bonding of permanent magnets, such as in the assembly of a permasyn motor, for example.

CLAIM FOR PRIORITY

This application claims priority to International Application No.PCT/DE00/00171 which was published in the German language on Aug. 3,2000.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a one-component epoxy resin adhesive, and inparticular, to a one-component epoxy resin adhesive which is elasticatedwith silicone rubber and whose open joint time can be adapted.

BACKGROUND OF THE INVENTION

In many sectors of industry, workpieces are already fastened or joinedexclusively by adhesive bonding. This is typically at the expense ofconventional fastening techniques such as welding, soldering, threadedunion, or the like. The continually growing fields of use demandadhesives which in turn are required to satisfy a wide variety ofrequirements.

DE 195 38 468 A1 discloses a one-component adhesive with a favorableprofile of properties. However, its highly reactive UV initiator doesnot allow an industrially practical open joint time, and its lack ofelasticating particles also renders it unsuitable for extensive (i.e.,large surface area) bonds.

SUMMARY OF THE INVENTION

The invention relates to a one-component epoxy resin adhesive which iselasticated with silicone rubber and whose open joint time can beadapted to whichever production process it is used in via the nature andconcentration of the photoinitiator and/or the duration and/or intensityof UV activation. This adhesive is preferentially suitable for automatedadhesive bonding processes and may also be used with advantage foradherents of large surface area having opposite thermal expansioncoefficients. It is suitable, for example, for adhesive bonds ofpermanent magnet materials, such as in the assembly of permanent magnetson metal pole plates and/or solid steel poles in machines withpermanent-magnet excitation.

The invention provides an adhesive and a method of adhesive bondingwhich can be used to realize industrially practical open joint times andextensive bonds.

Furthermore, the use of adhesives which form an elastic adhesive bond ison the increase, especially when the bonded union produced is to bethermally stable over a wide temperature range, since on heat-inducedexpansion or shrinkage of the workpieces the adhesive bond ought tocompensate differences in length by virtue of its elasticity.

The invention provides a one-component adhesive comprising:

A) 5-90% by weight of a cycloaliphatic epoxy resin component,

B) 10-94% by weight of an epoxy-functional silicone rubber,

C) 0.05-5% by weight of a ferrocene-based UV initiator,

D) 0.1-5% by weight of a thermal initiator,

E) 0.05-1.5% by weight of an adhesion promoter,

F) 0.1-10% by weight of a highly disperse silica,

G) 0-3% by weight of spacers, and

H) 0-70% by weight of filler.

The invention further provides a method of two-dimensionally adhesivelybonding two workpieces, which comprises:

applying a film of an adhesive to one of the two workpieces or to bothworkpieces,

setting a desired open joint time of the adhesive by duration-andintensity-specific UV irradiation and/or heat treatment and/or IRirradiation, for the purpose of positional fixing, and

joining the two workpieces,

thermally aftercuring the adhesive.

DETAILED DESCRIPTION OF THE INVENTION

A defined open joint time ensuring secure positioning of the workpiecesis made possible by the nature and concentration of the photoinitiatorand by the choice of intensity and duration of UV activation.

After the workpieces have been joined, the adhesive, adapted to themanufacturing sequence, may be cured at room temperature until itbecomes strong enough to handle, thereby enabling further processing ofthe resultant assembly without extra mounting-in an automated process,for example.

By briefly heating, such as infrared heating, the open joint time mayalso be significantly shortened and the handling strength increased as aresult. Ultimate strength is then attained in downstream thermal curing.

The open joint time of such an adhesive is easily controlled inaccordance with the specific requirement by the concentration ofinitiator in the adhesive, the duration and/or intensity of UVirradiation, and the temperature of the workpieces, adhesive film and/orsurroundings.

The open joint times which can be set vary from a number of seconds toseveral hours.

Because of the incorporated elastomer particles, the adhesive and themethod are also suitable for joining parts of large surface area,including for example solid workpieces having opposite thermal expansioncoefficients.

The adhesive preferably comprises a ferrocene-salt-based UV initiator,which brings about an open joint time of between 30 s and 3 h dependingon the concentration and on the duration and/or intensity of UVactivation. Moreover, it preferably comprises the epoxy-functionalsilicone rubber in the form of particles, which result in surprisinglyhigh elasticity of the adhesive bond.

For complete curing in circumstances where in some cases no UV lightreaches the bond, or where the amount which does reach it is inadequate,such as in the case of thick films and/or shadow regions, for example,the adhesive comprises a thermal initiator.

Constituent A of the adhesive is a cycloaliphatic epoxy resin, e.g., aring-epoxidized diepoxide such as the cycloaliphatic diglycidyl etherused in an amount of from 5 to 90% by weight, preferably from 5 to 50%by weight.

Component B is an epoxy-functional silicone rubber and is present in theadhesive in an amount of from 10 to 94% by weight, preferably 50-90% byweight, in particular 70-90% by weight. The epoxy-functional groupsincorporate the silicone rubber chemically into the polymer matrix. Thisrules out separation of these particles, which have an average size of0.1-3 μm.

Constituent C of the adhesive comprises a ferrocene-salt-based UVinitiator which on UV exposure undergoes photolysis and in doing soreleases acid cations which catalyze the epoxide polymerization. Thereaction rate is dependent on its concentration and on the processingtemperature employed. Preference is given to usingcyclo-pentadienylisopropylbenzeneiron(II) hexafluorophosphate.

Constituent D of the adhesive is a thermal initiator based, for example,on a thiolanium salt, such as benzylthiolanium hexafluoroantimonate, andconstituent E is a customary adhesion promoter, such asglycidyloxypropyltrimethoxysilane, for example.

The further constituents F, G, and H are a highly disperse silica, suchas Aerosil, ceramic or glass beads in order to produce a defined jointgap, and common fillers, such as quartz flour, for example.

Accordingly, the adhesive is curable by the dual principle: the curingreaction is initiated by UV irradiation tailored in its intensity andduration to the desired open joint time. Ultimate strength, includingespecially in the regions hidden by shadow from the UV light, isachieved with a subsequent thermal curing process.

In one advantageous embodiment of the method, an adhesive bond isproduced as follows: one or both of the work-pieces to be joined is orare provided with an adhesive film, preferably in a film thickness ofbetween 10 and 500 μm. The adhesive film is exposed to UV light for from1 to 60 s, depending on the open joint time required. At a UV initiatorconcentration of 0.5% by weight, an intensity of 40 mW/cm², and anexposure period of from 30 to 60 s, the resulting open joint time (i.e.,the period of time within which the adhesive remains liquid) is from 1to 2 hours.

Within the open joint time, the adherends are joined and aligned inaccordance with a template. After the open joint time has expired, theadhesive solidifies and, given the indicated exposure data, reachespracticable handling strength after 3 hours, with a shear strength of 2N/mm², which makes it possible to continue handling the assembly withoutusing holding means. In order to give the bonded assembly the maximumstrength, a thermal aftercure is performed.

Here, the thermal initiator that is present in the adhesive guaranteesfull and even curing of the bond site after 2 hours at 150° C.

Where more than two workpieces are to be bonded, further workpieces maybe attached and fixed following the fixing of at least one bond site.There is no need for a fixing means for the workpieces which are appliedfirst. Even in the case of bonds on sharply inclined, curved or evenoverhanging surfaces, fixing by UV irradiation is possible, and after anappropriate open joint time or after the bonded parts have attained ahandling strength the relative position of the bonded workpieces remainsunchanged even in the course of heating at the curing temperature. Thefixing is sufficiently stable and positionally accurate even whenadditional forces act on the bonded or fixed workpieces. This case isobserved, for example, with the adhesive bonding of magnets which, atshort distances, exert forces of magnetic attraction and repulsion onone another.

Following thermal aftercuring, the adhesive, and/or the bond siteproduced with the method using the adhesive, exhibits an ultimatestrength of >3.5 N/mm², measured on a rare-earth permanent magnetmaterial of large surface area, produced by powder metallurgy, namely“VACODYM”, in an adhesive bond with iron at 150° C. This level ofstrength is also maintained after storage at 150° C. for several weeks.An assembly thus produced is suitable for use in the temperature rangefrom −40° C. to 180° C.

The adhesive is applied at room temperature by means of dispensertechnology or knife-coating technology in a film thickness of 10-500 μm,preferably 70-150 μm, and with particular preference from 100 to 125 μm,on either one of the workpieces or both workpieces.

In order to set a defined joint gap it has proven advantageous to addspacers such as glass beads, for example. It is especially practical inthis case to use glass beads having a diameter corresponding to thetarget size of the joint gap. For example, by adding glass and/orceramic beads having a diameter of about 100-125 μm it is possible toproduce an adhesive film thickness and a joint gap having this order ofmagnitude.

The glass and/or ceramic beads may be incorporated into the adhesivebefore it is applied to one of the workpieces or may be scattered ontothe applied adhesive bed on the workpiece during the open joint time.

The amount of spacers in the adhesive is advantageously from 0.5 to 5%by weight. An amount of from 0.75 to 3, and in particular of about 1% byweight, based on the total adhesive mass, has been found particularlyadvantageous.

The adhesive bonding method is suitable for joining a large number ofvery different substrates. One field of use of the invention is, inparticular, the adhesive bonding of a permanent magnet element to aferromagnetic material, such as an iron pole, comprising a solid blockor a layered stack of metal plates, in an electrical machine. High bondstrength is also achieved, however, on glass, plastic, ceramic, andmetal.

In the text below the adhesive is illustrated with reference to aworking example (all components are, individually, availablecommercially):

A) 10.0 g of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate

B) 87.5 g of silicone-elastomer-modified epoxy resin

C) 0.5 g of cyclopentadienylisopropylbenzeneiron(II) hexafluorophosphate

D) 1.5 g of S-benzylthiolanium hexafluoroantimonate

E) 0.5 g of glycidyloxypropyltrimethoxysilane

F) 0.5 g of highly disperse silica

G) 1.0 g of glass beads

H) 12 g of quartz flour

The adhesive bonding method of the invention has the advantage thatjoints can be implemented continuously without complicated holdingmeans. The parameters for the open joint time, adjustable via the natureof the UV initiator, the concentration of the UV initiator, and theduration and/or intensity of UV activation, are to be chosen so that thesetting of the handling strength fits into the desired manufacturingprocess and the further handling of the workpieces to be bonded can becarried out without additional mounts.

An advantage with the initiator system of the adhesive of the inventionis that both the photoinitiator and the thermal initiator trigger acationic polymerization; i.e., the curing of the adhesive produces auniform, stress-free network. A further advantage is that thermal curingmay be performed independently of the point in time after the UVactivation. The ultimate strength obtained is always the same,irrespective of whether there is an open joint time of 1 minute or 3hours or even a storage period of 3 days.

What is claimed is:
 1. An adhesive, comprising: A) 5-90% by weight of acycloaliphatic epoxy resin component; B) 10-94% by weight ofepoxy-functional silicone rubber; C) 0.05-5% by weight of aferrocene-based UV initiator; D) 0.1-5% by weight of a thermalinitiator; E) 0.05-1.5% by weight of an adhesion promoter; F) 0.1-10% byweight of a highly disperse silica; G) 0-3% by weight of spacers; and H)0-70% by weight of filler.
 2. The adhesive as claimed in claim 1,wherein the ferrocene-based UV initiator iscyclopentadienylisopropylbenzeneiron(II) hexafluorophosphate.
 3. Theadhesive as claimed in claim 1, wherein the thermal initiator isbenzylthiolanium hexafluoroantimonate.
 4. The adhesive as claimed inclaim 1, wherein the adhesive adhesively bonds adherends of largesurface area having opposite expansion coefficients.
 5. A method oftwo-dimensionally adhesively bonding two workpieces, comprising:applying a film of an adhesive to at least one of the two workpieces;setting a desired open joint time of the adhesive by duration andintensity-specific UV irradiation and/or heat treatment and/or IRirradiation, for positional fixing; joining the two workpieces; andthermally aftercuring the adhesive; wherein said adhesive comprises: A)5-90% by weight of a cycloaliphatic epoxy resin component; B) 10-94% byweight of epoxy-functional silicone rubber; C) 0.05-5% by weight of aferrocene-based UV initiator; D) 0.1-5% by weight of a thermalinitiator; E) 0.05-1.5% by weight of an adhesion promoter; F) 0.1-10% byweight of a highly disperse silica; G) 0-3% by weight of spaces; and H)0-70% by weight of filler.
 6. The method as claimed in claim 5, whereinat least one additional workpiece is attached to the two workpieces andfixed before thermal curing.
 7. The method as claimed in claim 5, inwhich at least one of the workpieces is solid.
 8. An adhesively bondedassembly of at least two workpieces joined by an adhesive, the adhesivecomprising: A) 5-90% by weight of a cycloaliphatic epoxy resincomponent; B) 10-94% by weight of epoxy-functional silicone rubber; C)0.05-5% by weight of a ferrocene-based UV initiator; D) 0.1-5% by weightof a thermal initiator; E) 0.05-1.5% by weight of an adhesion promoter;F) 0.1-10% by weight of a highly disperse silica; G) 0-3% by weight ofspacers; and H) 0-70% by weight of filler.
 9. The assembly as claimed inclaim 8, wherein workpieces comprising a magnetic material are joined toa metallic substrate.
 10. The assembly as claimed in claim 8, whereinthe assembly operates in the temperature range from about −40° C. toabout 180° C.